ARM MP3/AAC Player Quote: "...This is my MP3/AAC player project. The difference to most other players is that decoding is not done on a specialized IC (like VS1001), but directly on the microcontroller. It can play MP3 (all bit rates) and AAC (up to ~256 kbps) from SD card in real time. The project is work in progress..."

This is what has been thought of: a simple processor that still suppports A/D converters and USB. A good candidate appears to be PIC16C765 Cost is approx. 5 USD.

Using an ARM processor just to convert some samples per second into USB appears to be overkill to me on first though. But the PIC is just one of the options to considered. A drawback appears that the ADC resolution is just 8 bit. This is quite coarse for some sensors. 12 bit would be better...

PIC16C745: July 25, 2003 My Very First USB Peripheral Quote: "...For example, I wanted to change the number of buttons and add a couple of LEDs. To do this, you need to build new HID report descriptors and there you run into problems with incomprehensible standards documentation and inconsistent operating systems implementations...One last item that fits in under "firmware" better than anywhere else is the issue of vendor IDs. Like PCI and other "plug and pray" bus systems, your device presents a vendor ID and a product ID so that the host operating system can locate appropriate drivers. Unlike those other systems, there is no "experimental" vendor ID or apparently any way to get a legitimate vendor ID other than paying the USB Implementers Forum a minimum of $1500 every couple of years...If you want a host application of your own talking to a collection of custom peripherals you've designed, though, things are much harder..."

I recommend an AVR or PIC.
Since they have no USB Interface, they are both very cheap and are easy to use.
For programming an AVR all needed tools are in the most distros repositorys (gcc-avr, binutils-avr, avrlibc, avrdude).
As USB interface, i recommend using an FTDI USB/Serial changer. They work very fine also with 1MBit Baud.

Every AVR has an integrated I²C, SPI and UART (Serial) Interface. You can connect 8 analog sensors, you have some Hardware Interrupts, Timers, PWM-Outputs,...

ARM MP3/AAC Player Quote: "...This is my MP3/AAC player project. The difference to most other players is that decoding is not done on a specialized IC (like VS1001), but directly on the microcontroller. It can play MP3 (all bit rates) and AAC (up to ~256 kbps) from SD card in real time. The project is work in progress..."

This is what has been thought of: a simple processor that still suppports A/D converters and USB. A good candidate appears to be PIC16C765 Cost is approx. 5 USD.

Using an ARM processor just to convert some samples per second into USB appears to be overkill to me on first though. But the PIC is just one of the options to considered. A drawback appears that the ADC resolution is just 8 bit. This is quite coarse for some sensors. 12 bit would be better...

PIC16C745: July 25, 2003 My Very First USB Peripheral Quote: "...For example, I wanted to change the number of buttons and add a couple of LEDs. To do this, you need to build new HID report descriptors and there you run into problems with incomprehensible standards documentation and inconsistent operating systems implementations...One last item that fits in under "firmware" better than anywhere else is the issue of vendor IDs. Like PCI and other "plug and pray" bus systems, your device presents a vendor ID and a product ID so that the host operating system can locate appropriate drivers. Unlike those other systems, there is no "experimental" vendor ID or apparently any way to get a legitimate vendor ID other than paying the USB Implementers Forum a minimum of $1500 every couple of years...If you want a host application of your own talking to a collection of custom peripherals you've designed, though, things are much harder..."